This invention relates to variable loss networks and in particular to such networks which are controlled by active circuit elements in response to an external control signal.
In a number of the prior art circuits, the output is controlled by controlling the gain of the output amplifier. A problem with such arrangements is that the gain may not be linear over the control range and further the impedance of the circuit output might change as the gain changes.
Other alternate ways which have been used to effect a constant output for a fixed gain amplifier, even though the input signal is varying over a fairly wide range, is to insert a variable impedance network between the input signal and the input to the fixed gain amplifier which impedance is responsive to the variations in the signal level. One such arrangement includes a resistor and a diode with the current through the diode varied in accordance with a control signal. The control signal is a direct current generated from a control device which is responsive to the output circuit voltage of the fixed gain amplifier. A disadvantage of such an arrangement is that the control current for the diode must be isolated both from the signal input circuit and from the input to the fixed gain amplifier. In addition the control current may become excessive. If the range of control is in the 60 dB range, for example, the control current can approach several milliamperes at high input levels. Further, a separate bias circuit is required for biasing the fixed gain amplifier and this adds to the current drain from the power source which can be a disadvantage in telecommunications circuits where low power can mean longer operating time in subscriber circuits where battery power is employed. These problems are solved by the circuit of the instant invention.